Realtime radiation exposure monitor and control apparatus

ABSTRACT

An apparatus for measuring radiation exposure in realtime by measuring the charge flowing in an external circuit when radiation creates electron hole pairs, and thus allows discharge, of a duo-dielectric detector.

RELATED APPLICATIONS

This is a divisional of co-pending application Ser. No. 022,989, filedon Mar. 22, 1979.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to apparatus and methods used toobtain image information from modulation of a uniform flux. Morespecifically, the present invention relates to methods and apparatusused to produce a radiograph by producing a latent image in aphotoconductor sandwich structure and then optically reading out thelatent image.

2. Background of the Prior Art

a. Facsimile and Vidicons

Facsimile systems modulate an electrical signal in response to lightthat is reflected from a small portion of an image. Facsimile requiresproduction of a real image.

Vidicon tubes store a latent image as a charge distribution pattern in asemiconductor target. This latent image is scanned by an electron beamand the current variation induced by the different charges on separateportions of the target comprises a video signal. Vidicons require use ofa high vacuum and precise focusing of an electron beam that must beshielded from external electric and magnetic fields.

b. Conventional Radiographic Methods

Latent images stored in silver halide film or selenium xeroradiographicplates must be chemically or powder cloud developed to produce realimages. Use of calcium tungstate crystals or a high atomic weight gasfor image intensification degrades image quality and still requiresexposures of from 1 to 5 R per typical clinical mammogram. Powder clouddevelopment of latent xeroradiographic images requires high differentialcharge densities on the xeroradiographic plate to attract and holdpowder particles prior to fusing. This high charge differential isgenerated by x-rays, or ions, impinging on the surface of a chargedselenium plate. The higher the differential charge needed to produce animage, the more X-ray exposure is required.

The reader is directed to the following publications for detaileddiscussion of this prior art.

XERORADIOGRAPHY, J. W. Boag, Phys. Med. Biol., 1973, Vol. 18, No. 1, pp.3-37; Principals of Radiographic Exposure and Processing, Arthur W.Fuchs, 1958, Chapter 14, "X-Ray Intensifying Screens," pp. 158-164.

U.S. Pat. No. 3,860,817; Reducing Patient X-Ray Dose During FluoroscopyWith an Image System.

U.S. Pat. No. 3,828,191, Gas Handling System for ElectronradiographyImaging Chamber.

U.S. Pat. No. 3,308,233, Xerographic Facsimile Printer Having Light BeamScanning and Electrical Charging With Transparent Conductive Belt.

Recent research by the present inventors and others indicates that adetailed latent image can be created in a xeroradiographic plate by verylow levels (under 10 mR) of X-ray exposure. At this exposure levellatent image is present, but its associated electrostatic field is notof sufficient intensity to produce a real image by the powder cloudmethod of image development. For details of this research see, GrantApplication, "Radiomammography With Less than 150 mR Per Exposure,"available from the Department of Experimental Radiology, M. D. AndersonHospital, Houston, Texas 77025.

c. Semiconductor Art

The prior art of reading charge storage patterns out of multilayersemiconductor sandwich structures lies largely in the area ofelectronics, especially exotic computer memories.

Charge patterns on certain MOS structures can be "read out" using aphoton beam. See, Imaging and Storage With a Uniform MOS Structure,Applied Physics Letters, Vol. 11, Number 11, pp. 359-361. Thistechnology functions by modifying a depletion layer and then chargingthe layer to saturation. The few micron thick depletion layer is theonly active structure.

Electric fields can also be impressed across two separablephotoconductive insulating films in pressure contact that are prechargedto the same polarity. See, Increasing the Sensitivity of XerographicPhotoconductors, IBM Technical Disclosure Bulletin, Vol. 6, No. 10,1964, page 60.

IBM has also developed an exotic charge storage beam addressable memorycomprising a semiconductor sandwich wherein the semiconductor is totallyinsulated from both electrodes in the sandwich. See, IBM TechnicalDisclosure Bulletin, Vol. 9, No. 5, 1966, pp. 555-556. This deviceallows data readout by shifting charge population to one side or theother of the insulated semiconductor.

Finally, some theoretical work has been done on the behavior of lightsensitive capacitors, see, Analysis and Performance of a Light SensitiveCapacitor, Proceedings of the IEEE, April 1965, p. 378.

d. Objects of the Present Invention

It is an object of the present invention to provide a method andapparatus capable of replacing conventional photographic andradiographic films.

Another purpose of the present invention is to provide an X-ray sensingsystem capable of quickly producing radiographic images while exposingthe sensed patient or object to lower radiation dosage than has beenpractical using prior art systems.

A further purpose of the present invention is to provide an X-raysensing system whose output is an analog or digital video signal thatmay be selectively displayed on a television monitor, recorded on film,or directly stored or processed in a computer for image enhancement orpattern recognition.

Yet a further purpose of the present invention is to provide a novelmethod and apparatus for converting a charge distribution on asemi-conducting surface to a modulated electric signal.

Another purpose of the present invention is to provide an apparatus andmethod that combines the edge enhancement effect of xeroradiography witha low patient dose level.

Yet still another purpose of the present invention is to provide a novellow noise method and apparatus for reading out a latent image stored asa pattern of electrical charges by selectively discharging said chargesin the presence of a reverse biased electric field.

Yet another purpose of the present invention is to provide an X-raysensing system that is capable of directly timing X-ray exposure.

A final purpose of the present invention is to provide an apparatuscapable of converting a latent image to a modulated electric signal thatis simple and inexpensive to build and operate.

SUMMARY OF THE INVENTION

The present invention is an X-ray film replacement comprising a sandwichdetector structure; a method of converting the latent radiographic imagestored by such a detector to an electric video signal; an apparatuscapable of practicing the method and a diagnostic X-ray system using theapparatus.

The detector sandwich comprises a conductive back plate overlain by aphotoconductive layer, for example, a layer of amorphous selenium havinga high effective dark resistivity. This high effective dark resistivitymay be accomplished by forming a blocking diode layer between thephotoconductor and the conductive back plate. The photoconductor is, inturn, overlaid by an insulator which is covered by a transparentconductive film. The structure may be shielded to suppress externalelectrical noise.

The method of operating the detector structure requires that thedetector be charged to a high potential, preferably while thephotoconductive layer is flooded with light to decrease its resistance.After the light is extinguished, the conductive back plate andtransparent film are shorted together, causing surface chargeredistribution. The detector is then reverse biased by reversing theelectric potential placed across it. A subsequent X-ray exposure forms alatent image by selectively discharging part of the detector's surfacecharge. This discharge current may be integrated to control theexposure.

The detector need not be flooded with light, but without the light theresistance of the photoconductor will be higher and the detector willtake longer to charge. The latent image can, also, be formed by exposingthe uncharged detector structure to an X-ray flux while itsphotoconductor layer is biased by an electric field.

The detector is read out by being raster scanned by a beam of photons.These photons create electron-hole pairs in the photoconductor, whichallow a portion of the the photoconductor's surface charge to dischargethrough an external electric circuit. The signal amplitude in thisexternal circuit at any given time is a function of the intensity of thelatent image in the portion of the photoconductor being irradiated withphotons at that time, i.e., it is a video signal.

The apparatus for practicing the method comprises means for charging thephotoconductor; means for exposing it to X-rays to form a latent imageand means for outputing this latent image as a video signal.

Since the present invention is essentially a replacement for X-ray film,it can be used with any conventional source of radiation. A typicaldiagnostic X-ray system using the present invention also includes meansfor storing, transferring and processing the video signal to produceclinical images useable by a radiologist.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged cross-sectional view of a portion of oneembodiment of the multilayered photon detector apparatus taught by thepresent invention;

FIG. 1A is a schematic electrical diagram illustrating the electriccircuit analog of the structure shown in FIG. 1;

FIG. 2 is a view of the structure shown in FIG. 1 being flooded withphotons;

FIG. 2A is a schematic showing the electric circuit analog of FIG. 2;

FIG. 3 is a view of the detector structure after it is charged, floodedwith photons and the surface charge has been redistributed;

FIG. 3A is the electrical schematic analog of the detector as it isshown in FIG. 3;

FIG. 4 illustrates the detector structure of a preferred embodiment ofthe present invention as it is used in an X-ray system;

FIG. 5 is a greatly enlarged schematic cross-sectional view of a portionof FIG. 4 illustrating the effect of a modulated X-ray flux on thedetector;

FIG. 6 is a schematic view of the detector graphically illustrating thechange in detector surface charge caused by X-ray exposure;

FIG. 7 is a schematic view of the detector apparatus taught by thepresent invention being read out by a thin, scanning photon beam, anoutput wave form of the video electric signal is illustrated togetherwith a schematic representation of the surface charge of the detector;

FIG. 8 is a highly enlarged schematic cross-sectional view of a smallportion of the detector structure shown in FIG. 7 illustrating theelectron hole production mechanism in the photoconductor layer used bythe present invention;

FIG. 9 shows a partially cut-away view of an apparatus adapted topractice the preferred embodiment of the present invention;

FIG. 10 is a simplified diagram showing the major parts of the apparatusshown in FIG. 9;

FIG. 11 is a schematic block diagram illustrating a diagnostic X-raysystem constructed according to a preferred embodiment of the presentinvention;

FIG. 12 shows a cross-sectional view of a clinical apparatus forconducting mass chest radiographic screening using the presentinvention;

FIG. 13 is a highly enlarged schematic cross-sectional view of anembodiment of the present invention used in the apparatus shown in FIG.12.

FIG. 14 is a graph illustrating the theoretical maximum chargeobtainable from an experimental detector structure per unit area of thedetector as a function of the voltage applied across its photoconductivelayer. FIG. 14 also shows a plot of specific points representingexperimental results from tests run on this experimental detectorstructure for a given mylar thickness and selenium thickness.

FIG. 15 is a graph illustrating the voltage across the selenium layer ofa theoretical detector in kilovolts as a function of the value of C2, infarads, for that detector.

FIG. 16 is a graph illustrating the charge, in coulombs collected by theexperimental detector structure per unit of exposure as a function oftotal exposure. The values plotted in FIG. 16 were obtained using aBakalite shutter;

FIG. 17 illustrates the same information as FIG. 16, but the data wasobtained using a PVC shutter;

FIG. 18 is a graph showing the efficiency of the present invention, asexpressed in coulombs of charge obtained per roentgen of exposure,plotted against supply voltage for various levels of total exposure.This figure illustrates the increased efficiency of the presentinvention at low radiation levels;

FIG. 19 is a graph that plots the total charge collected by anexperimental embodiment of the present invention as a function ofapplied voltage across its photoconductive layer. FIG. 19 illustratesincrease in latitude of the present invention's detector that isobserved as applied voltage across the photoconductor is increased;

FIG. 20 is a functional block diagram of an experimental prototypesystem of the present invention.

FIG. 21 is a schematic diagram showing a portion of the detectorstructure used as a radiation exposure detector automatically timing theexposure.

FIG. 22 is an electrical schematic showing the reversed biased mode ofdetector operation.

FIG. 23 is an electrical schematic of the preamplifier used inexperimental system 3.

FIG. 24 is a schematic diagram of the X-position interface module usedin experimental system 3.

FIG. 25 is the characteristic curve of the detector structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, multilayered photon detector apparatus 10 has first electrode12 conductively connected to a first conductive layer 14. Aphotoconductive layer 16 is in physical and electrical contact withplate 14 which may be made of aluminum or any other conductor. An oxidelayer 11 may act as a blocking contact between layer 16 and plate 14.Layers 14, 16, 11 may be provided by using a commercial xeroradiographicplate. Transparent insulating layer 18 overlays and may be integrallyaffixed to selenium photoconductor layer 16. A transparent conductivelayer 20 may be integrally affixed to and overlies insulator 18. Theselayers may be made by vapor deposition or by adhesively bonding theindividual components together. Layer 20 is electrically connected tolead 22.

DISCUSSION OF EXPERIMENTAL SYSTEM #1

The experimental plates used to test the present invention as early asMarch 1976 were made from conventional xeroradiographic plates. Theseplates are made for mamography by the Xerox Company. To make the presentinvention's detector, a plate is first cut to the correct size and thencovered with mylar. The conductive coating, which is either Nesa glassor a few angstroms of gold, is placed on the mylar.

In the preferred embodiment of the present invention discussed in thissection of the specification, the aluminum backing plate 14 isapproximately 1/10 of an inch thick and the layer of amorphous selenium16 is approximately 150 microns thick. The physical properties ofselenium are listed in Table I. The transparent insulator 18 is mylar.The transparent conductor 20 may be Nesa glass, a thin film of metaldeposited directly on the transparent insulator 18, or a plastic filmwith a conductive coating, i.e., gold covered mylar. The entirestructure 10 may be made by depositing successive layers of selenium,mylar and a thin film of metal onto an aluminum plate. Assembly may beaccomplished by vapor deposition, sputtering, or any other techniqueuseful to deposit even thickness films. This technology is welldeveloped in the art of semiconductor electronics and glassmanufacturing.

The thickness of the selenium layer must be selected to maximize quantumefficiency of the detector. This optimum thickness will be a function ofthe photoconductor's characteristics, the potential at which thedetector is operated and the energy of the X-ray photons or otherparticles to which the detector is exposed that act to discharge it.

Basically, the thicker the selenium, the more it interacts with a givenenergy of exposing radiation and the more electron-hole pairs a givenquantity of radiation produces. Conversely, as the selenium layer ismade thinner the electric field acting on these electron-hole pairsbecomes stronger (the same potential over less distance). Thus a verythin layer of selenium would not interact with the exposing radiationenough to produce electron-hole pairs while a very thick layer ofselenium would result in the production of lots of electron-hole pairs,but they would all recombine when the exposure stopped.

The optimum thickness of the photoconductor layer of the detector willdepend on the characteristics of the photoconductor and the energy ofthe X-ray photons it is designed to sense. In the experimentalembodiments of the present invention applicants have found that from onehundred microns to four hundred microns thickness of amorphous seleniumis optimum for the X-ray photon energies commonly used in diagnosticradiology.

There are several facts the inventors discovered experimentally thatshould be mentioned.

When several experimental prototype detector structures were built underlaboratory "clean room" conditions, they worked poorly. Regular aluminumbacked selenium-xeroradiography plates covered with gold covered mylarplastic and assembled with optical cement under less than idealconditions work very well.

It appears that a very thin aluminum oxide layer between the seleniumand the aluminum acts as a blocking contact, i.e., a diode, to retainthe positive charge on the surface of the selenium. This blocking layerhas a blocking potential that must be overcome for current to flowthrough the contact.

Also, when the experimental detectors built by the inventors werescanned fast, a higher signal strength output was obtained than whenthey were scanned slowly. Since the light intensity was the same, thefaster scan was predicted to result in lower signal strength. Applicantscannot yet explain this phenomena, but, by pulse modulating their lightsource for between 2 nanoseconds and 10 microseconds on and off, thiseffect can be used to increase signal strength from the presentinvention.

Finally, repeated use of the prototype plate resulted in the developmentof "artifacts," i.e., lines and trash on the image. Repeated use alsocaused an overall loss in detector sensitivity. It was accidentallydiscovered that heating the prototype plate for a short time with a heatgun used to shrink plastic tubing removed these artifacts and otherwisegenerally restored the detector's performance.

In FIG. 1, R1 represents the resistance of the photoconductive layerwhen it is flooded with light. R2 represents the resistance of thephotoconductor layers when it is in the dark. C2 represents theelectrical capacity manifested by the charge separation across thetransparent insulator 18. One charge polarity resides on the transparentconductor 20 and the other on the surface of selenium, which is inimmediate contact with insulator 18. C1 represents the electricalcapacity of the detector measured between the aluminum conductor 14 andthe selenium surface 16. The selenium is thus the dielectric material ofC1.

FIG. 1A shows an electrical schematic illustrating an electronic analogof detector sandwich structure 10 shown in FIG. 1. R1, R2, C1 and C2 areshown schematically connected in FIG. 1A as electrical symbols. Switch24, which is shown open, is used to represent the photoconductive natureof selenium layer 16 of FIG. 1. Selenium has a dark resistance ofapproximately 10¹⁶ ohm-centimeter. A portion of this resistance iscaused by a blocking contact formed at the Se-Al interface. When photonsstrike the photoconductor its resistance is lowered because the photonscreate electron hole pairs that carry current. This lower resistance isillustrated schematically in the electrical analogs contained in thisspecification by a closure of switch 24 leaving only residual resistanceR1, which represents the forward resistance of the blocking contact inseries with the resistance of the selenium layer when it is flooded withphotons. For a current to flow when the photoconductor is flooded withlight the blocking potential of blocking contact 11 must be overcome.

                  TABLE I                                                         ______________________________________                                        Physical Properties of Selenium                                               (2,3)                                                                         ______________________________________                                        Atomic Number            34                                                   Density (g cm.sup.-3)    4.25                                                 Dielectric Constant      6.6                                                  Resistivity at 20° C. (˜cm)                                                               10.sup.13 -10.sup.16                                 Thermal Conductivity at 20° C.                                         (Wcm.sup.-1 K.sup.-1)    2 × 10.sup.-3                                  Optical Band Gap (2V)    2.3                                                  Photo Response Edge (AE) 4600                                                 K absorption Edge (KeV)  12.7                                                 Mobility of Holes (cm.sup.2 s.sup.-1 v.sup.-1)                                                         0.14                                                 Mobility of Electrons (cm.sup.2 s.sup.-1 v.sup.-1)                                                     5 × 10.sup.-3                                  Energy for Production of Charge                                               Carriers (eV)            7*                                                   ______________________________________                                         *W = 2.67 × E.sub.g + 0.86 eV in which E.sub.g is the optical band      gap                                                                      

FIG. 2 illustrates detector 10 being flooded with photons 26.Simultaneously a negative high voltage, i.e., 2,000 volts is applied toterminal 22, hence to transparent conductor 20.

FIG. 2A, the electrical schematic analog of FIG. 2, shows switch 24closed by light 26. The -2,000 volt potential, Vs, is impressed acrosselectrodes 12 and 22.

FIG. 3 shows the semiconductor detector sandwich 10 after light 26 hasbeen extinguished and leads 12 and 22 shorted together. The resultingpositive surface charge on the amorphous selenium layer is illustrateddiagrammatically by dotted line 28. This surface charge is uniformlydistributed over the entire surface.

FIG. 3A is FIG. 3's electrical analog. The detector is in darkness, soswitch 24 is illustrated as open. Diode 15 and battery 13 represent theblocking contact and potential, respectively, of blocking junction 11.The electric charge originally present on capacitor C2 has redistributedso that a portion of that charge is present on capacitor C1, the exactportion being dependent upon the ratio of C1 to (C1+C2).

FIG. 4 shows the charged detector illustrated in FIG. 3 used as an imagereceptor structure for an X-ray image. Uniform X-ray flux 30 isgenerated by a convenient source of radiation, such as an X-ray tube,not shown. The detector will work with any radiation source capable ofgenerating electron-hole pairs in the photoconductor. This uniformphoton flux encounters and interacts with object 32, which may be anyobject of interest. Object 32 is placed directly over detector 10. Forsimplicity, object 32 is represented as an oblate spheroid of uniformdensity.

FIG. 5 shows a greatly enlarged schematic view of a portion of detectorstructure 10 of FIG. 4. Modulated X-ray flux 34 from the radiation thathas passed through object 32, creates electron-hole pairs 36 in seleniumlayer 16.

FIG. 6 shows detector 10 of FIG. 4 after the X-ray exposure has beencompleted. The detector is now storing a latent image. The modulatedsurface charge comprising the image is schematically illustrated by ahumped dotted line 38. This dotted line represents the change inpotential created by the electron-hole pairs generated by the X-rayexposure.

FIG. 7 shows detector 10 having a latent image stored as a modulatedsurface charge 38. A thin beam of light 40 is shown scanning the surfaceof photodetector conductive layer 16 in a regular raster pattern. In theexperimental system number 1 embodiment of the present invention, thisthin scanning light beam is produced by a He-Cd laser.

It should be understood that the photon beam need not be coherent. Itmay be of any frequency capable of creating electron-hole pairs inphotoconductive layer 16 of detector 10.

Electrode 41 is connected to electrical ground and electrode 43 carriesa video electric signal whose wave form is a function of modulatedsurface charge 38 in detector 10 scanned by light beam 40.

Arrow 42 indicates the direction of movement of scanning light beam 40.Output wave form 44 indicates the voltage variation of the output videosignal obtained by said beam's scanning the latent image.

FIG. 8 is a schematic, highly enlarged cross-sectional view of a portionof the detector structure being scanned by light beam 40 of FIG. 7.Light beam 40 penetrates the transparent conductor and transparentinsulator to generate electron-hole pairs 36 in that portion of seleniumlayer 16 irradiated by the beam.

Functionally, detector 10 operates by placing a uniform surface chargeon selenium layer 16 and then selectively discharging part of thissurface charge by X-ray exposure to form a latent image.

As shown in FIG. 2, when light photons flood detector 10, seleniumphotoconductor layer 16 becomes conductive. If, as shown, a -2,000 voltpotential is applied between the aluminum backplate (at ground) and thetransparent conductor 20 (at -2,000 volts potential), then the detectorwill charge as a capacitor. FIG. 2A illustrates how this charge isapplied to the system. Light flood 26 increases conductivity in theselenium layer 16. The applied voltage V_(s) is -2,000 volts. Thiscauses a charge to build up across capacitor C2, which represents thecapacity between the surface of the selenium in contact with thetransparent insulator and the transparent conductor of detector 10.Capacitor C2 thus charges to potential V_(s) of 2,000 volts.

After capacitor C2 has charged to potential V_(s), the light flood isremoved and detector 10 is stored in darkness. In the absence ofphotons, amorphous selenium layer 16 becomes nonconductive and its darkresistivity in synergistic combination with blocking contact 11 preventsthe surface charge on the selenium from leaking off. This condition isillustrated by FIGS. 3 and 3A. After detector 10 is in darkness,terminals 22 and 12 are shorted together. This causes the electriccharge on capacitor C2 to redistribute between capacitor C2 and C1, eachof which is then charged to a potential of one-half V_(s), i.e., 1,000volts, if C1=C2. Capacitor C1 represents capacitance between the surfaceof the photoconductor layer 16 and the aluminum backing 14. This 1,000volt surface charge is distributed uniformly over the selenium layer'ssurface. This surface charge is maintained across the selenium layer bythe high dark resistivity of the selenium in conjunction with the nowreverse biased blocking junction at the Al-Se interface. The effectiveblocking potential of this junction has been experimentally determinedto be about 150 to 300 volts in the experimental systems discussedbelow.

Resistance R1, representing resistance of the selenium layer when it isexposed to light, is relatively low compared to its dark resistance R2.

Each photon striking the photoconductor, depending on its energy,generates a specific number of electron-hole pairs withinphotoconductive layer 16 (see Table II). Each electron-hole pairdischarges a portion of the surface charge at the spot it is generated.

FIG. 4 illustrates how this effect is used to impress a latent image onthe detector.

FIG. 4 is a cross-section schematic view of the photon detectorstructure taught by the present invention being used as the imagereceptor in an X-ray system.

A uniform flux of X-ray photons 30 is modulated, i.e., partiallyabsorbed, by its passage through an object being X-rayed 32. Themodulated X-ray flux then strikes detector 10.

Detector 10 has a 1,000 volt surface charge, as was illustrated bycharge 28 in FIG. 3, impressed on selenium layer 16. As the X-rayphotons strike the selenium layer, they generate electron-hole pairs.For monochromatic X-rays the number of electron-hole pairs generated isa direct function of the number of X-ray photons striking the seleniumdetector layer.

Some of the X-ray photons in uniform flux 30 are absorbed by object 32.Thus the modulated flux striking photoconductive layer 16 containsinformation about the internal structure of the object being x-rayed.This information is contained in the number of photons striking eachportion of the detector.

FIG. 5 illustrates how this modulated X-ray flux creates a modulatedsurface charge on the surface of the detector. Modulated X-ray flux 34generates electron-hole pairs 36 in selenium layer 16 of detector 10.The number of electron hole pairs generated at each point on the surfaceof the detector is a function of the number of X-ray photons impingingon the photoconductive layer. Each electron-hole pair 36 discharges aportion of the surface charge impressed on the photoconductive layer atthe point where it is generated. The greatest number of electron-holepairs will be generated where the X-ray photons from uniform flux 30strike the detector without any absorption from the object beingx-rayed. A lesser number of X-ray photons will strike thephotoconductive layer under the object being X-rayed. The radioopacityof the object being X-rayed is thus reproduced in the surface charge ofthe detector structure after X-ray exposure.

FIG. 6 schematically illustrates the detector structure's modulatedsurface charge after the X-ray exposure described in connection withFIGS. 4 and 5.

The surface charge is lower where no object absorbed X-ray photons. Theobject being X-rayed, which is assumed to be uniformly opaque to X-rays,absorbs a portion of the uniform X-ray flux. This results in thegeneration of fewer electron hole pairs under the object and a highersurface charge on the detector under the object being X-rayed. This isrepresented by modulated surface charge dotted line 38.

FIG. 7 illustrates schematically the readout method used to extract anelectric video signal from the modulated surface charge which forms thelatent image on a detector structure taught by the present invention.

The detector structure is kept in darkness and a thin beam of light,preferably generated by a helium-cadmium laser, is scanned in a regularraster pattern across the surface of photoconductor 16.

As the scanning light beam 40 moves in direction illustrated by arrow42, it produces a small moving spot on the surface of photoconductivelayer 16. The size of this spot determines the resolution of the finalimage. It is therefore desirable that this spot size be kept small.

As laser beam 40 scans the surface of photoconductive layer 16, itgenerates electron-hole pairs as is illustrated in detail by FIG. 8.These electron-hole pairs are mobile within the photoconductive layerand discharge a portion of the surface charge. In the preferredembodiment of the present invention the electrons move toward thepositively charged selenium surface of the detector sandwich structure10. The voltage drop produced across the resistor connected betweenground connection 41, which is attached to transparent conductor 20 andvideo output electrode 43, which is attached to aluminum conductivebackplate 14, will be a function of the intensity of the surface chargeat the point where the laser beam generates the electron-hole pair.

Scanning laser beam 40 thus produces a modulated electric signal 44 atoutput electrode 43. The voltage of this output electric signal will bea function of the surface charge present at the spot where the laserbeam strikes the photoconductive layer of the detector. The currentpresent in the output circuit is a function of the frequency andintensity of the scanning light beam and is a further function of thespeed with which the light beam scans the surface of the photoconductor.Video signal 44 can be electronically processed to produce an image thatreproduces the latent image found in the surface charge of thephotoconductive layer of the detector.

Depending on the intensity of the surface charge, the beam scanningspeed, and the frequency and intensity of the light beam used, thesurface charge may be repetitively scanned several times by the beam oflight.

Table II lists the values for the thickness of the selenium layer, theX-ray photon energy, the fraction of this energy absorbed by theselenium layer, quantum conversion between X-ray photons andelectron-hole pairs, dark resistance of the selenium and the totalreceptor area of detector 10 used in calculating the operatingparameters of a sample detector constructed according to the preferredembodiment of the present invention.

                  TABLE II                                                        ______________________________________                                        Assumed Values                                                                ______________________________________                                        Thickness of Selenium Layer                                                                          100 microns                                            Energy Radiation       21 keV                                                 Absorbed Fraction of photons                                                                         85%                                                    Quantum Conversion     3000 electron/                                                                photon                                                 Surface Voltage        1000 V                                                 Photon Flux/Roentgen   5 + 10.sup.9 cm.sup.-2                                 Dark Resistivity       10.sup.16 Ω cm                                   Total Receptor Area    560 cm.sup.2                                           ______________________________________                                    

Table III lists the calculated capacity of the detector sandwichstructure, the charge density present in this structure when the surfacepotential is 1,000 volts, the number of elementary charges present persquare centimeter in the structure and the dark resistance and darkcurrent of the detector at a thousand volts charge.

                  TABLE III                                                       ______________________________________                                        Calculated Values                                                             ______________________________________                                        Capacitance           5.8 × 10.sup.-11 F/cm.sup.2                       Charge Density at 1000 Volts                                                                        5.8 × 10.sup.-8 Coul/cm.sup.2                     Number of Elementary Charges                                                                        3.6 × 10.sup.11 el/cm.sup.2                       Dark Resistance       10.sup.14 Ω/cm.sup.2                              Dark Current at 1000 Volts                                                                          10.sup.-11 A/cm.sup.2                                   ______________________________________                                    

Using the values from these tables it is possible to calculate operatingparameters for a typical detector system constructed according to thepreferred embodiment of the present invention.

If detector 10 has an area of 560 square centimeters and is exposed touniform flux of 21 keV X-rays resulting in an exposure of Np X-rayphotons per unit area, then the total number of electron hole pairsgenerated will be 3,000 times N_(p). Information theory requires thatthree times as many electron hole pairs be generated as are present atthe statistical noise level of the system for the latent image to bedetectable.

There are approximately 5 times 10⁹ photons per square centimeter perroentgen of X-ray exposure at 21 Kev. Thus the minimum detectable latentradiation exposure that will produce a detectable image in the seleniumsandwich structure taught by the preferred embodiment of the presentinvention (for a 560 square centimeter detector) is approximately 23micro-roentgens per exposure. This may be thought of as the theoreticallower limit of exposure to generate a 10 line pair per millimeter imagewith the present invention.

Turning now to a practical example, if the X-ray dose of a patient scanis 100 mR and at the thinnest section of the X-rayed object 50% of thisradiation is absorbed by the sample, then exposure at the detector atthe brightest portion of the image amounts to 50 mR while the lowesttheoretical detectable dose, as was discussed above, amounts to 23 μR.This theoretically yields a latent image represented by modulation ofthe surface charge present on the detector at a ratio between thebrightest and the darkest areas of the image of approximately 2,000.

The example discussed above shows that the detector structure taught bythe present invention is theoretically capable of recording an imageusing a selenium photoconductive layer with a resolution of 10 linepairs per millimeter and a brightness range of over 2,000 after a totalX-ray exposure of only 100 mR. An example of how this latent image canbe read out of the detector follows.

FIG. 9 shows a partially cut-away view of an apparatus capable ofreading out the image whose generation was discussed above.

Light-tight housing 46 contains a light source 48 aligned so as toproject a light beam 50 onto the front of a scanning means 52 which maybe a rotating multi-sided mirror. Mirror 52 is mounted on an axis 54which is operably attached to a scanning motor 56. The mirror axis andscanning motor are affixed to a platform 58. This platform is mounted onan axis 60 which is orthogonal to axis 54. Axis 60 is mounted at one endto first upright support 62 and its other end to second upright support64. The end of axis 60 is swingably mounted in second upright 64 and isoperably attached to stepping motor 66.

The entire scanning means comprising the mirror and associatedpositioning components is available from Texas Medical Instruments,Inc., 12108 Radium, San Antonio, Tex. 78216.

Mirror 52 and its associated scanning mechanism is located at one end ofhousing 46. A multilayered photon detector apparatus 10 is removablyinserted through a light-tight opening 68 into a holder at the other endof housing 46 such that deposited transparent conductive layer 20 facestoward mirror 52.

Light source 48 is preferably a Helium-Cadmium laser such as a Liconixmodel #402 laser with an optical modulator. This laser produces a beamof intense light at approximately 4,400 angstroms.

Scanning motor 56 rotates multi-sided mirror 52 on axis 54 to causelight beam 50 to scan horizontally across the surface of detector 10.This causes spot 70 to intersect the selenium photoconductive layer aswas discussed in connection with FIGS. 7 and 8 above. Each time spot 70moves from the left to the right side of detector 10, stepping motor 66moves platform 58 through sufficient arc to deflect spot 70 vertically1/20th of a millimeter. This stepping-scanning function may becontrolled mechanically or electrically. By convention, spot 70 beginsscanning the surface of detector 10 at its upper left hand corner. Afterraster scanning the entire surface of the detector, the scanningmechanism may be programmed either to turn off or to rescan the plate.

FIG. 10 is a simplified schematic view of the scanning readout mechanismshown in FIG. 9. Light beam 50 from laser 48 reflects off of thepolygonal surface of multi-sided mirror 52 as that mirror rotates onaxis 54. This causes light beam 50 to generate flying spot 70 whichmoves across the surface of detector structure 10. As was discussed inconnection with FIGS. 7 and 8, above, this causes a video signal to begenerated across the detector structure.

Theoretically, photons from light source 48 have a wave length ofapproximately 4,400 angstroms. The quantum yield of these photons andselenium approaches unity when field strength nears 10⁵ volts percentimeter. In a 150 micron selenium plate, such as is used in thepreferred embodiment of the present invention, this corresponds to athousand volts surface potential.

It is possible to pulse the laser to decrease the scanning time requiredto obtain an image from the detector structure. In the present example,the laser beam may be pulsed for a 100 nanoseconds period with 3 or 4microsecond spacing between pulses. This illuminates each image elementsufficiently to read out a latent image stored in the detector. Thepractical advantage of such a regime is to permit much shorter samplingtimes. It may also result in higher signal strengths.

FIG. 11 is a block diagram of a radiographic X-ray system taught by thepreferred embodiment of the present invention. Laser 72 produces a smallintense beam of light 74 which is expanded by optical objective 76 andlenses 78 and 80 into a reasonably wide parallel beam 82. Focusingmirror 84 reflects beam 82 on the reflective surface 86 of scanningmechanism 88. Scanning mechanism 88 is substantially the same as thescanning mechanism discussed in connection with FIG. 9 above. Thisscanning mechanism may be any apparatus capable of moving the laser beamover the detector and film. For example it may be a set of computercontrolled mirrors or holographic optics.

Beam 82 projects a small flying spot 90 onto the surface of detectorstructure 10. Detector structure 10 was discussed in detail inconnection with FIGS. 1 through 8, above.

Detector structure 10 is electrically connected by lines 92 and 94 tocooled amplifier 96. Amplifier 96 may be a low noise amplifier.Amplifier 96 is connected by line 98 to lines 100 and 102 whichelectrically connect the output of cooled amplifier 96 to signalprocessor 104 and second signal processor 106, respectively. Signalprocessor 104 is connected by line 108 to beam modulator 110. Laser 112produces an intense beam of coherent light 114 which is modulated bybeam modulator 110 and spread by objective 116 in lenses 118 and 120 tocoherent modulated beam 122. Modulated beam 122 is focused by focusingmirror 124 onto a second mirror surface 126 of optical scanner 88. Thissurface may be another surface of the same scanner or may be a separateoptical scanning system.

Mechanical movement of scanning system 88 causes beam 122 to form flyingspot 128 which scans the surface of recording medium 130. This processallows the very low intensity latent image on detector structure 10 tobe electrically amplified by cooled amplifier 96 and signal processor104 and then to be rewritten as an intensified image on a photographicor xeroradiographic plate 130.

The electrical output of cooled amplifier 96 also is fed to signalprocessor 106 by lines 98 and 102. The output of signal processor 106 isconnected by line 132 to a digital computer 134. Computer 134 is used todigitally store and manipulate the information content imparted to it bythe electrical signal produced by cooled amplifier 96 by way of signalprocessor 106. The images may be stored on magnetic tape or disc filesand can be manipulated within the computer by algorithms for image edgeenhancement or pattern recognition for automated diagonsis.

The output of signal processor 106 also goes by line 136 to mass storagesystem 138.

Mass storage system 138 contains a high resolution display tube 140which produces an analog image 142 which is focused by focusing optics144 onto film plane 146 of a mass film storage system 148. This massfilm storage system may be a 35 or 70 mm cassette system.

The type of radiographic image processing and storage system describedin connection with FIG. 11 is especially useful for interfacing massradiographic data acquisition equipment with the central computingfacilities of a large hospital complex. Digital storage of theradiographic images allows the data to be accessed by a remoteradiologist with speed and precision. Computer-based pattern recognitionalgorithms allow inexpensive gross screening of large patientpopulations for radiographic anomalies. The system's ability to rewriteinformation obtained at very low radiation dosages onto conventionalxeroradiographic plates or films permits the system to protect thepatient while interfacing with presently existing radiographic datastorage formats.

As will be discussed further below, the radiographic data comprising thelatent image on detector structure 10 may be read out in real time toproduce a video image which may be studied by the radiologist.

FIG. 12 shows a mass screening system constructed according to thepreferred embodiment of the present invention. The embodiment shown inFIG. 12 could be used for real time read out of chest films in a massscreening program. X-ray source 150 produces a uniform flux of X-rays152. Uniform flux 152 passes through and is modulated by patient 154.The modulated X-ray flux impinges onto multilayered photon detectorapparatus 156.

FIG. 13 shows a highly enlarged cross-sectional view of a portion ofdetector sandwich structure 156 used in FIG. 12.

Detector structure 156 is like detector structure 10 described in FIGS.1 through 8, above, except that it is positioned so the modulated X-rayflux impinges on the selenium photoconductive layer after it passesthrough the aluminum backplate. Additionally, a thin insulating layer158 has been added to the outer surface of the aluminum supportstructure to electrically insulate the aluminum layer from patient 154.

Detector sandwich structure 156 is mounted within light-tight housing160. Housing 160 also contains a means for scanning the detectorstructure with a thin beam of light, i.e., laser scanning system 162which projects light beam 164 in a raster pattern onto the back side,i.e., the conductive transparent coating side, of detector 156. Base 166of housing 160 contains control electronics 168 and signal electronics170. The entire housing and base may be mounted on legs 172 to raisedetector structure 156 to chest height of patient 154.

Control means 168 comprises electronics necessary to control thescanning pattern of laser scanner 162 so as to cause light beam 164 toscan the photoconductive side of detector 156 in a regular rasterpattern. The control electronics also controls X-ray source 150 andsynchronizes exposure and readout operations.

Signal acquisition means 170 comprises electric amplifiers connected tothe video output of detector 156. Signal electronics 170 amplifies thedetector output and provides a video output capable of operating videodisplay means 174, which may be a high resolution video display monitor.

Operationally, the mass screening system shown in FIG. 12 is a smallportable unit. Patient 154 steps up to the unit and presses his chestagainst thin insulating layer 158 over the aluminum back plate ofdetector structure 156. X-ray source 150 is then turned on by thecontrol means and irradiates the patient with under 100 mR of X-rayradiation. As is shown in FIG. 13, this modulated X-ray flux penetratesthin insulating layer 158 and the aluminum backplate of detector 156.The X-ray flux then generates electron hole pairs in the seleniumdetector structure as was discussed in connection with FIGS. 2 through6, above. The effect of X-rays being absorbed in the selenium withconsequent neutralization of surface changes, is a displacement currentthat flows as C2 redistributes its charge over C1. When this current isintegrated over time it will reach a preset value. Control circuitrywill then terminate the X-ray exposure. Use of such circuitry insuresimages of equivalent quality regardless of the thickness of the patient.The above device or operation is called "automatic photo-timing" or"automatic exposure control".

Referring again to FIG. 12, as soon as the exposure is completed,control electronics 168 causes laser scanner 162 to scan light beam 164across the surface of the selenium photoconductive layer in a regularraster pattern as was described in connection with FIGS. 7 through 10,above. This causes detector 156 to produce a video signal containingimage information. This video signal is fed to signal electronics 170where it is amplified to provide a video output to TV monitor 174.

Such a system may be used to make a clinical radiograph of any bodypart, including breast, chest, head, etc.

The detector itself, being a replacement for film in a radiographicsystem, can be used with intensifying screens. One embodiment of thedetector structure would use a phosphor, preferably a high Z rare earthphosphor, in place of insulating layer 18 in FIG. 1. The X-rays strikingthe phosphor would generate light which would create ion-hole pairs inphotoconductive layer 16.

Derivation of the Characteristic Curve of the Detector

This section refers to the derivation of the curve shown in FIG. 25.

When the experimental system is exposed to light, a total charge ofQ_(L) coulombs will flow in the external circuit where ##EQU1## Exposureto X-radiation prior to such a light exposure would simply reduce Q_(L)proportional to the X-ray exposure dose.

One may compute the charges placed in motion due to absorption of X-rayphotons by first defining the photo-generation efficiency n after Fender##EQU2## where ε is the energy deposited within the selenium in ev dueto absorbed photons, n is the number of ions which neutralize the chargeon the surface of the absorber, and E is the average electric fieldacross the absorber. Also we note, ##EQU3## where Q_(x) is the surfacecharge neutralized in coulombs per square centimeter, A the area of theabsorber in square centimeters and e the electronic charge incoulombs/ion-pair. The energy absorbed may be calculated as

    ε=fkXA and dε=fkAdX                        (4)

where f is the fraction of X-ray photon absorbed in the selenium, k isthe energy fluence in air per roentgen exposure and X is the exposure inroentgens. Also, since the electric field is that which exists withinthe selenium, ##EQU4## where Q*₁ is the instantaneous charge existingacross the selenium, C1 the capacitance of the selenium layer, and d₁the thickness of the selenium. Substituting (3) (4) and (5) into (2) wehave ##EQU5## From (1) we may express Q*₁ as ##EQU6## substituting inr6), we have the differential equation, ##EQU7## the solution of whichis ##EQU8## Then the total charges placed in motion when illuminating apixel which has absorbed X roentgens of X-ray is,

    Qsig=Q.sub.L -Q.sub.R                                      (10)

from (1) and (10), ##EQU9## Equation (12) is of the form described byBoag in which he states that the radiographic discharge of electrostaticplates is exponential.

FIG. 19 is a plot of Q_(R) versus V_(s) for various doses ofX-radiation. From such data we may solve equation (8) for η, thephotogeneration efficiency. From FIG. 19 we also note the emperical datapredicts a loss of charge which may be due to recombination; however, itmay also be indicative of deep hole traps within the selenium thatestablish a residual potential below which the system cannot bedischarged.

Description and Discussion of Three Methods of Charging the DetectorCharging Method #1

To review briefly, the semiconductor sandwich-like structure,illustrated by FIGS. 1 and 1A above, appears as a pair of capacitors inseries. C2 is the capacitor formed in the experimental system by Nesaconductive glass and the surface of the selenium proximate the Nesa. Amylar insulating layer acts as a dielectric. C1 is the capacitor formedby the surface of the selenium and the aluminum substrate, the seleniumphotoconductor acting as the dielectric. Because photoconductors are notdielectrics when they are illuminated, C1 will exist only when it is inthe dark. This ignores such things as deep hole traps, dark currents,etc., but they will be discussed in turn.

Referring to FIG. 1, when a voltage is applied between terminals 22 and12, a current will flow charging capacitors C1 and C2 to voltages V1 andV2 respectively, where:

    V.sub.s =V.sub.1 +V.sub.2                                  (1)

In which case: ##EQU10## Where V_(s) is the supply voltage, and##EQU11## If the selenium layer is illuminated while there is apotential between terminals 22 and 12, capacitor C1 becomes a conductorand allows additional current to flow to C2. This additional charge Q₁is the charge initially residing on C₁. That is, since

    Q.sub.1 =C.sub.1 V.sub.1                                   (4)

substituting (3) into (4), ##EQU12##

The initial voltage across C2 is determined as in (2). The increase involtage across C2 due to C1 becoming conductive is: ##EQU13## thus thetotal voltage across C₂ is now (2) plus (6) or ##EQU14##

Because of (1), V₁ is now found to be equal to zero. The charge acrossC₂ is now the total Q or

    Q.sub.T =V.sub.s C.sub.2                                   (8)

If the illumination is removed, this voltage distribution will persistuntil the potential is removed from terminals 22 and 12 are connectedtogether. C₂ will now partially discharge into C₁ resulting in an equalvoltage appearing across C₁ and C₂ since they are now essentially inparallel. This voltage will be ##EQU15##

Note that V₁ and V₂ are equal, however, of opposite polarity.

It will also be noted that no supply voltage V_(s) is in the circuitduring this redistribution of charge. The above operation has resultedin placing a charge on the surface of the selenium much as isaccomplished in the Xeroradiographic procedure. In the presentinvention, however, the charge has been applied to a selenium surfacethat is physically inaccessible.

The system is now ready for exposure to X-rays. An integrating ammeteror a coulomb meter is placed in the circuit between C₁ and R₂ such thatthe current, or net charge movement due to radiation exposure may bemeasured. This flow of current is proportional to exposure and thus canbe used to control the exposure. This is advantageous because it allowsthe exposure time to be controlled by the amount of radiation actuallyreaching the detector.

Alternatively, the surface of capacitor C2, which is a thin conductivefilm, may be etched to form a small island as shown in FIG. 21. In FIG.21 coulomb meter 2101 may be connected between island electrode 2103 andthe aluminum substrate 2105. During X-radiation exposure meter 2101 willaccumulate a charge which may be used to photo-time the exposure. Thisis advantageous because regardless of the thickness of the patient, theX-ray machine will stay turned on until the correct amount of charge iscollected by the detector, thus insuring a properly exposed image.Several of these isolated islands may be etched in the plate and used asexposure meters.

When the detector is exposed to radiation, electron-hole pairs arecreated within the selenium which neutralize the charge across C₁. Asthese charges are neutralized, the charge on C₂ redistributes. Thisredistribution maintains the two capacitors at the same potential. Thusthe total charge flowing through the external circuit, i.e., throughload resistor R₂, is the initial charge placed on C₂ alone. From (9) wecan obtain the voltage across C₂, i.e., the total charge on C₂ prior toX-ray exposure: ##EQU16##

Equation (10) is the total charge that will move if the system is notexposed to radiation. Should X-ray exposure occur to create a latentimage, a portion Qx of the charges on C₁ will be neutralized. The chargeremaining on C₂ will be: ##EQU17## Where Q_(T) =C₂ V_(s) as in (8). IfQ_(X) =0, then Equation (11) becomes (10).

This charge can be accumulated by a coulomb meter and would be afunction of the number of electron-hole pairs generated by the X-raysabsorbed in the selenium. When the detector is scanned by the laserbeam, each pixel, i.e. the area illuminated by the laser beam at rest,is sequentially discharged. Functionally, C₁ is made conductive wherethe light shines. When C₁ becomes conductive, the charge on C₂ can flowthrough the external circuit. The current which flows through theexternal circuit is a function of the latent image stored in thephotoconductor.

As the laser beam scans across C₁, the IR drop appearing across resistorR₂ is a video signal.

Charging Method #2

Consider the same sandwich-like detector structure in a circuitconfiguration analogous to the one shown in FIG. 22.

Structurally, three-position switch 2201 is connected to one side of theplate structure 2203 illustrated as capacitor C₁ and C₂ in series. Oneside of coulomb meter 2205 is connected to the other side of platestructure 2203. The other side of coulomb meter 2205 is connected to oneside of resistor 2207. The other side of resistor 2207 is connected tothe positive side of battery 2209; directly to terminal 2211 and to thenegative side of battery 2213. The positive side of battery 2213 isconnected to terminal 1 of three-position switch 2201. Terminal 2211 isconnected to terminal 2 of switch 2201. The negative side of battery2209 is connected to terminal 3 of three-position switch 2201.

Three position switch 2201 is first placed in the position 1. Thevoltages across C₁ and C₂ are the same as shown in FIG. 1 and 1A. If theselenium is now illuminated, C₁ becomes conductive as before and V₂=V_(s) as in (7). If the illumination is now removed this voltagedistribution again persists. Moving switch 2201 to position 2 results inthe charge on C₂ being distributed over both C₁ and C₂ as before with V₁=-V₂ as given in (9).

If switch 2201 is moved to position 3, a voltage will exist across C₂which will be that ratio of V_(s) due to capacitance C₁ and C₂ less thevoltage due to trapped charges (9), i.e., ##EQU18## since V₁ +V₂ =V_(s)

we find ##EQU19## that is, the voltage across the selenium is now twicethat obtained in the other charging method #1.

Now the total charge on C₂ is from (13) ##EQU20##

Under these conditions if the ratio of C₂ /C₁ +C₂ is near unity, then V₂will be opposite in polarity to V₁ as well as at a lesser potential.

If we assume C₂ =10 C₁, the ratio C₂ /(C₁ +C₂)=10/11, also assumingV_(s) =100 V, from (14) ##EQU21##

Thus the Q₂ as shown in Equation (15) represents a charge opposite tothat on C₁ and that of the voltage supply V_(s).

Upon exposure to radiation and/or subsequent read-out by the laser, thetotal Q_(Sig) moving through the external circuit will be that residingon C₂ initially, plus that required to charge C₂ to a voltage of V_(s)in the opposite polarity. Thus ##EQU22##

If the detector has been exposed to X-rays then, ##EQU23## here, asbefore, Q_(X) are those charges neutralized by the electron-hole createdin the selenium by absorbed X-ray photons.

In summary, use of charge method #2 results in doubling the voltageacross the selenium without doubling the voltage applied across mylarcapacitor C₂. This allows capacitor C₂ to have a thinner dielectric andthus a higher capacitance. Increasing the value of C₂ relative to thevalue of C₁ brings the fraction C₂ /(C₁ +C₂) nearer to unity, whichfurther increases the detector's signal output. A further advantage ofdoubling the voltage across the selenium is that the increased electricfield strength decreases electron-hole pair recombination in theselenium and increases the efficiency of ion-pair collection. Thisincreases the efficiency of the detector.

Increasing C₂, or rather making C₁ as small as possible, has twobeneficial results:

A. Reducing the capacity of C₁ is accomplished by increasing thethickness of the selenium layer in the detector, which increases thedetector's X-ray absorption efficiency.

B. Reducing C₁ while using the highest possible value of C₂ reduces thetotal capacitance of the detector, since C₁ and C₂ are in series. Thislower total capacitance increases the scan speed that the invention canachieve. This is important because the prior art only teaches thesegmenting of a plate into small sections to avoid this largecapacitance.

EXPERIMENTAL SYSTEM #2 The Experimental Apparatus

To evaluate the operational characteristics of the detector and readoutmeans of the present invention, a model detector as in FIG. 1 was builtand mounted on an optical bench which contained a He-Cd laser,collimating lens and field stop. The C₂ was formed of approximately 5mil mylar plastic coated with a few angstroms of gold, i.e., to wherethe surface resistance of the mylar is equal to 20 ohms per square(layer 20, FIG. 1). The mylar was bonded to the selenium layer withoptical cement. The laser was pulsed via a pulse generator. Chargemethod #2 was used as described above. The detector was loaded withvarious values of resistances in accordance with the applied voltage. Anamplifier with a gain of 1000 was used to condition the detector'soutput signal prior to display on a storage oscilloscope. A schematic ofthe amplifier is given in FIG. 23. From photographs taken of the signalson the oscilloscope's screen the total area under the discharge curvewas determined via planimeter to estimate the charge set in motion bythe laser. This procedure was repeated after exposure to measureexposure to X-ray radiation to determine the number of coulombs ofcharge discharged by the X-ray exposure.

Experimental Results

Experiments were designed to determine if detector and readout means ofthe present invention performed as predicted. Using Equation (17), thetotal possible signal, in coulombs per square centimeter, was calculatedassuming Q=0, C₁ =3.69×10⁻¹¹ and C₂ =2.93×10⁻¹¹ farads/cm². FIG. 14 is agraph of the number of coulombs theoretically predicted by such a modelas a function of supply voltage applied across the detector. Also shownon FIG. 14 are several experimentally measured values of chargescollected from experimental system #2, which was constructed and testedin the experimental radiology laboratory of M. D. Anderson Hospital inHouston, Texas.

In experimental system #2 the selenium thickness was measured as 150microns. Five mil mylar was employed as the second dielectric. Allcharges calculated in FIG. 14 assumed an active area or pixel size of0.3 cm² and represent total discharge of the pixel.

As can be seen from FIG. 14, the correlation of calculated and measuredcharge collected is very good if one assumes a loss of 3.93×10⁻⁹coulombs. This departure from the theoretically obtained value may bedue to incomplete discharge caused by the existence of a blockingcontact at the selenium/aluminum interface. The fact that the empericalexperimental data does not extrapolate to zero may also imply that asthe plate nears total discharge, fewer charges are collected becausemore recombination takes place at lower field strength.

FIG. 15 is a graph illustrating the electric field strength across theselenium layer of the detector as a function of mylar thickness, i.e.C₂, for a given selenium thickness and various supply voltages appliedby charge method #2. This graph is useful in predicting the dynamicrange of the system in coulombs for various combinations of C₂ andV_(s). In experimental system #2, C₁ is fixed at 3.69×10⁻¹¹ farads/cm²by the 150 micron thickness of selenium. Referring again to FIG. 15, ifC₂ is 8×10⁻¹¹ farads/cm² and V_(s) at 2000 V, we find the voltageapplied to the selenium will be 2500 V, using charge method #2, and thedynamic range of the detector will be 1×10⁻⁷ coul/cm². This means thatfor each square centimeter of plate illuminated, a total of 1×10⁻⁷coulombs will flow in the external circuit. X-ray exposure to produce alatent image would decrease this total.

The detector's sensitivity to X-ray exposure is of great importance;that is, the number of coulombs of charge set in motion and collectedper absorption of an X-ray photon. FIG. 16 is a graph showing thecoulombs of charge collected per roentgen exposure as a function oftotal exposure (measured in MAS (Milliamperes X seconds) applied to anX-ray tube).

The X-ray system employed with experimetal system #2 was a SiemensMammomat designed for operation with a xeroradiographic system. TheX-ray unit was operated at 44 kVp and had a half-value-layer (HVL) ofapproximately 1.5 mm. of aluminum. The receptor was housed in a cassettemade of 1/16th inch PVC and had a Bakelite shutter.

FIG. 16 shows that the sensitivity (Q/roent.) of the present inventionis higher for higher applied voltages and that the invention isunexpectedly sensitive to low exposure levels. The present invention'sefficiency of collection, expressed in coulombs per roentgen, is muchhigher at low X-ray exposure levels than at high exposure levels.

FIG. 17 describes the same data as FIG. 16; however, the X-ray detectorcassette is equipped with a 1/16" thick PVC shutter. The highattenuation of PVC to the X-rays employed in these experiments was notappreciated at the time this data was collected.

FIG. 18 illustrates the efficiency of the present invention incoulombs/roentgen, as a function of supply voltage. Again the system isclearly more efficient at lower radiation levels. Table IV simplypresents the same data in tabular form.

                                      TABLE IV                                    __________________________________________________________________________    SUPPLY       200mAs         320mAs                                            VOLTAGE                                                                               Q(mc)                                                                              Bakelite                                                                           %  PVC %  Bakelite                                                                           %  PVC %                                     __________________________________________________________________________     600    .91 ± 0.10                                                                       --  --  .64nC                                                                            70.3                                                                              --  --  .63nC                                                                            69                                    1200   3.11 ± .60                                                                       2.58nC                                                                             82.9                                                                             2.38nC                                                                            76.5                                                                             2.80nC                                                                             90 2.88                                                                              92.6                                  1500   3.78 ± .30                                                                       3.20nC                                                                             84.6                                                                             2.80                                                                              74.0                                                                             3.40 89.9                                                                             3.26                                                                              86.2                                  2100   6.32 ± .52                                                                       5.19 82.1                                                                             3.5 55.3                                                                             5.74 90.8                                                                             4.0 63.3                                  2400   6.98 ± .19                                                                         -- -- 4.2 60.2                                                                              --  -- 5.6 80.2                                  2700   8.25 ± .21                                                                       7.14 86.5                                                                             4.6 55.7                                                                             7.69 93.2                                                                             6.0 72.7                                  __________________________________________________________________________     When a Bakelite shutter is used at 320 MAS and 2700 V supply voltage,     93.2% of all available charges transfer. This represents near total     discharge. Further, in view of the variance shown in Table IV in total     charges available, the above example does indeed represent total     discharges within experimental error.

FIG. 19 illustrates total charge collected as a function of voltageapplied across the detector. The curves diverge for the variousexposures as the applied voltage is increased. This can be interpretedas meaning that the latitude of the detector, i.e. its ability todifferentiate exposures of various levels, increases as applied voltageincreases. Optimum latitude can be determined as a trade off between thedynamic range of the system and the signal to noise (S/N) ratio of thedetector output signal.

Estimates of the values of C₁ and C₂ have been made based oncalculations of capacitance assuming selenium thickness of 150 micronsand a dielectric constant of 6.3; mylar thickness of 4.3 mils anddielectric constant of 3.5.

The experimental results allow certain conclusions to be drawn about thepresent invention:

(1) Use of charge method #2 has allowed use of increased voltage acrossthe selenium while maintaining a lower voltage across the mylar.

(2) For a given thickness of selenium, the dynamic range of the systemmay be varied by varying the mylar thickness and applied voltage.

(3) The system's dynamic range must necessarily be greater than therange in charges neutralized by radiation absorption, i.e. the x-rayscannot be allowed to completely discharge any part of the detector. Thisis necessary so as to maintain a field across the selenium even in areasof greatest radiation exposure. Doing so will maintain a good collectionefficiency.

It is empirically estimated that any one pixel should not be dischargedbeyond 50% of its initial charge. This, however, is only an estimate andhas yet to be derived theoretically or by optimalization procedures.

(4) Using the data collected, an example of the operationalcharacteristics of the system can be computed as follows. Assuming 44kVp, 320 MAS x-ray source and a 6 cm. lucite phantom, the maximumexposure to the receptor is 370 mR. When operating the invention at 2700V applied, 6.0×10⁻⁹ coulombs are placed in motion due to irradiating anarea of 0.3 cm². Given a load resistance of 1000Ω and readout time of4×10⁻⁶ sec. (corresponds to 125,000 hz band width), an amplifier gain of1000, and a pixel size of 50 micron, the output signal is: ##EQU24##

    E.sub.out =48 mv

System noise may be evaluated as amplifier noise plus Nyquist noise ofthe input resistance ##EQU25##

Using an amplifier with noise figure of 0.8 nano volt/√hz and 1.5×10⁵hz, ##EQU26##

Hence, the amplifier noise is much less than the Nyquist noise of theinput resistor. Therefore, signal to noise ratio is:

    S/N=(48./1.57)=30.5:1

this dynamic range will produce an image with approximately 9-10 shadesof gray. Here a shade of gray is defined as √2 increase in signal. Thatis,

    Shades of Gray=(2 log S/N/log 2)

EXPERIMENTAL SYSTEM #3

FIG. 20 is a block diagram that illustrates the major components of thethird experimental system built at the experimental radiology departmentof M. D. Anderson Hospital in Houston, Texas. This experimental systemwas built to test the entire integrated system of the present inventionand to provide a breadboard system for initial clinical evaluation. Thissystem is fully computerized.

As is shown in FIG. 20, microcomputer 2007, which is a SouthwestTechnical Products 6800, controls laser blanker 2015; 16-bit d/aconverter 2011; x-position interface module 2017; 12-bit d/a converter2009; and teletype 2005. Teletype 2005 is a model 33ASR. Sixteen-bit d/aconverter 2011 is an Analog Devices 1136 and 12-bit d/a converter 2009is an Analog Devices 1132. Laser blanking unit and laser 2015 are aLiconix model 4110 helium/cadmium laser.

Laser 2015 is in a light-tight housing, not shown, such as the onefunctionally illustrated in FIG. 9, above.

The photon beam from laser 2015 scans detector 2019. The output ofdetector 2019 is connected to the input of preamp 2021. The output ofpreamp 2021 is connected to scan rate converter 2003; d/a converter2023; and write laser modulator and laser 2025.

The output of d/a converter 2023 is an input to microcomputer 2007.Teletype 2005 is in two way communication with microcomputer 2007.

It would be appreciated by looking at FIG. 20 that laser 2015 is a readlaser adapted to scan detector 2019 and laser 2025 is a write laseradapted to write information out on a film or xerographic plate. Thistype of system is generally described in connection with FIG. 11, above.

Both the read and write laser beams are controlled by the microcomputer.The output of 12-bit d/a converter 2009 drives read laser x-positionscanner, controller and mirror transducer 2013. It also drives writelaser x-position scanner, controller and mirror transducer 2027.Similarly, 16-bit d/a converter 2011 drives both read laser y-positionscanner, controller and mirror transducer 2029 and write lasery-position scanner, controller and mirror transducer 2031. It willeasily be appreciated that the beams from both the read laser and thewrite laser scan in synchronization, although both of them need not beturned on at the same time or modulated by the same signal.

Functionally, the microprocesser accepts instructions from the teletype.These instructions operate on a computer program. A copy of thisprogram's listing is included with this application for the Examiner'sconvenience. It is the inventors' request that it be inserted in therecord so it will be available to the public as part of the File Wrapperof this application.

When the microprocessor addresses the 16-bit d/a converter andincrements the number stored in it, the y-position mirrors of bothlasers are deflected to a certain angle. In experimental system #3 thisangle is sufficient to move the spot that is illuminated on detector2019 50 microns. Thus the y-positioning of the laser beam on thedetector occurs in 50 micron increments. The program included with thisapplication allows specification of line sizing on the y-axis to 50,100, 200, or 400 microns. This translates to a resolution in the imageproduced by the system of 10, 5, 2.5 and 1.25 line pairs per millimeter.

The 12-bit d/a converter is not directly addressed by the computer. Thecomputer addresses a special interface that controls the 12-bit d/aconverter. This interface allows the computer to specify the startingand ending positions of the laser beam on the detector surface and alongthe x-axis. A schematic of x-position interface module 2017 is includedas FIG. 24. The computer then issues a start command to the interfacewhich enables a switch selectable clock to start incrementing the d/a toa specific start address. The clock continues to increment the d/a untilthe stop address is encountered, then the clock is disabled by theinterface. This x-position interface accomplishes two objectives. First,the data readout range can be varied. Secondly, different size detectorplates can be read out without overscanning them. This is especiallyvaluable on an experimental system.

Computer 2007 also controls laser blanking, which just means that itturns the laser on and off. The computer blanks the laser after eachx-axis scan is completed. The laser is then moved back across thedetector and turned back on for the next scan line.

To read out a plate using experimental system #3:

(1) The user specifies plate size and x-axis line spacing. The computerprogram will ask for these values.

(2) The x-axis interface is given the appropriate start and stopaddresses by the computer.

(3) The x-axis line spacing and total number of required steps isdetermined by the computer.

(4) The computer blanks the CRT and puts the scan rate converter intothe write mode.

(5) The laser is turned on by the computer.

(6) The computer instructs the interface to turn on the clock.

(7) The x-axis mirror sweeps the laser across the plate.

(8) The computer turns off the laser.

(9) The computer increments the y-axis d/a the required number of stepswhich moves the laser beam through the specified line spacing.

(10) The computer then repeats steps 5-9 until the entire plate has beenscanned.

(11) The computer puts the scan rate converter in the read mode and theimage is displayed.

The read laser x-position scan controller and y-position scan controllerboth feed into scan rate converter 2003. The scan rate converter allowsa relatively slow video image, such as that formed by experimentalsystem #3, to be interfaced with a television display. Scan rateconverter 2003 outputs a normal video signal to CRT display 2001, wherethe image may be viewed, moved about on the screen, or expanded toexamine detail.

The example and suggested systems illustrated and discussed in thisspecification are intended only to teach those skilled in the art thebest way known to the inventors to make and use their invention. Nothingin this specification should be considered as limiting the scope of thepresent invention. Many changes could be made by those skilled in theart to produce equivalent systems without departing from the invention.For example, it is possible to use a photoconductor other than seleniumas part of the detector sandwich. Holographic optics could be used inthe scanning system in place of the mechanical systems used in theexample. It may be desirable to alter the relative thicknesses of thevarious layers making up the sandwich structure or to provide fordifferent frequency of light beams to read out the radiographic image orto sense different x-ray potentials. The present invention should onlybe limited by the following claims and their equivalents.

I claim:
 1. An exposure measuring apparatus comprising:a multilayereddetector structure, in an electric potential field, said detectorcomprising a duo-dielectric sandwich including, a transparent conductinglayer overlying a transparent insulating layer which overlays a highresistance photoconductor layer which overlays a conductive layer; anexternal circuit connecting said transparent conducting layer and saidconducting layer; measuring means responsive to the flow of current insaid external circuit for measuring the flow of current in said externalcircuit when said duo-dielectric detector structure is irradiated byradiation capable of forming electron hole pairs in the photoconductivelayer of said multilayered detector structure.
 2. An apparatus as inclaim 1 wherein the flow of current is measured from only a part of saiddetector structure, said small part being electrically segmented fromthe greater portion of the detector.
 3. An apparatus as in claim 2wherein current flow is measured from several small segmented portionsof the detector.
 4. An apparatus as in claim 1, 2 or 3 including controlmeans responsive to said measuring means for stopping said irradiationwhen the total charge reaches a preselected limit.